This invention is in the field of digital printing, and is more specifically directed to image formation in electrophotographic printing.
Electrographic printing has become a prevalent technology in the modern computer-driven printing of text and images, on a wide variety of hard copy media. This technology is also referred to as electrographic marking, electrostatographic printing or marking, and electrophotographic printing or marking. Conventional electrographic printers are well suited for high resolution and high speed printing, with resolutions of 600 dpi (dots per inch) and higher becoming available even at modest prices. At these resolutions, modern electrographic printers and copiers are well-suited to be digitally controlled and driven, and are thus highly compatible with computer graphics and imaging. Examples of conventional printing machines with this capability include the DIGIMASTER 9110 network imaging system and the DIGIMASTER 9150i digital press, both available from Heidelberg USA, Inc.
A typical electrographic printer includes a primary image forming photoconductor, which may be a moving belt in large scale printers, or a rotating drum in smaller laser printers and photocopiers. The photoconductor is initially sensitized or conditioned by the application of a uniform electrostatic charge at a primary charging station in the printer. An exposure station forms an image on the sensitized photoconductor by selectively exposing it with light according to the image or text to be printed. The exposure station may be implemented as a laser, an array of light emitting diodes (LEDs), or a spatial light modulator. In modern electrographic printing, a computer typically drives the exposure station in a raster scan manner according to a bit map of the image to be printed. The exposing light discharges selected pixel locations of the photoconductor, so that the pattern of localized voltages across the photoconductor corresponds to the image to be printed.
At a developing or toning station in the typical electrographic printer, a developer roller or brush is biased to a bias voltage roughly at the primary charging voltage of the sensitized photoconductor prior to exposure. The biased developer roller or brush is loaded with toner, which is typically a mixture of a fine metallic powder with polyester resin and powdered dye, charged to the bias voltage. As the exposed photoconductor passes the developing station, toner is attracted to the discharged pixel locations of the photoconductor. As a result, a pattern of toner corresponding to the image to be printed appears on the photoconductor. This pattern of toner is then transferred to the medium (e.g., paper) at a transfer station. The transfer station charges the medium to an opposing voltage, so that the toner on the photoconductor is attracted to the medium as it is placed in proximity to the photoconductor.
The transferred toner is not permanently fixed to the medium at the transfer station, however. Conventional electrographic printers have a fusing, or fixing, station located downstream from the transfer station, at which the transferred toner pattern is fused to the medium. Conventional fusing stations apply heat and pressure to fuse the transferred toner to the medium, after which it travels to a finishing station in the printer for collating, sorting, stapling or other binding, and other finishing operations.
In order to permanently fix or fuse the toner material onto the medium using heat, the temperature of the toner material is elevated to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent into the fibers or pores of the receiving medium. The toner material then solidifies as it cools, bonding firmly to the receiving medium.
One approach to the thermal fusing of toner is to pass the receiver with its electrostatically adhered toner images between a pair of opposed rollers, at least one of which is heated. In a fusing system of this type, the receiving medium passes through a nip formed at the contact location between the opposed rollers, typically with the side of the medium having the toner pattern contacting the heated fuser roller. The toner pattern is thus heated by the roller as the medium passes within the nip. In typical conventional fusing stations, the fusing roller (i.e., the roller contacting the toner side of the medium) is coated with an silicon rubber or other low surface energy elastomer.
Because the toner pattern is made tacky by heat, though, there is a tendency for toner to be retained by the heated fuser roller, rather than penetrate into the receiver medium. If this occurs, the retained toner can transfer to the next receiver sheet as it is fused. This retained toner could also transfer to the opposed pressure roller while no sheets are passing through the fusing station. In either case, so-called “offset” image artifacts, as referred to in the copying art, can be formed on subsequent sheets.
To address this problem, a thin layer of a toner release agent, for example a silicone oil (e.g., polydimethylsiloxane), is applied to the fuser roller to form an interface between the roller surface and the toner pattern on the imaged medium, and act as a polymeric release agent. The relatively low surface energy of this low viscosity oil enables release of the fuser roller from the tackified toner, and from the receiver medium to which the toner pattern is bonded as it passes through the roller nip. This clean release prevents the toner from offsetting to the surface of the fuser roller. Typically, the release oil is applied to the surface of the fuser roller by a donor roller that is coated with oil provided by a supply sump. Examples of roller-based fusing stations utilizing a toner release oil applied by a donor roller are described in U.S. Pat. No. 6,190,771 and U.S. Pat. No. 6,517,346 B1, both incorporated herein by this reference.
In conventional printing machines, as evident from the above description, the release oil is applied over the entire surface of the roller, regardless of the size of the paper or other medium that is being printed. The release oil is effectively cleaned from the fuser roller by the medium itself as it passes through the roller nip, to the extent of the area of the fuser roller that contacts the medium. It has been observed, however, that excess release oil will tend to build up on the fuser roller at those regions that are outside of the paper contact area.
Modern printing machines are capable of printing on a wide range of paper sizes. It has been observed, however, in connection with this invention, that if a significant release oil buildup occurs after the printing of relatively small page sizes, the subsequent printing of larger sized media will cause some of the built-up release oil to be transferred from the fuser roller to sheets of the larger media. The presence of this release oil on the larger media will appear as an image artifact.
In addition, this oil residue has a tendency to attract other contaminants, such as paper fibers, dust, and the like, that are present in the printing machine. This effect is especially a problem in printers having a duplex mode, where the printed (on one side) sheet returns to the transfer and fusing stations to receive a printed image on its other side. It has been observed, according to this invention, that fuser release oil residue on a sheet that is returned to the transfer station will convey these contaminants to the transfer station. These transferred contaminants not only result in additional image artifacts on subsequent sheets, but can also eventually foul the interior of the entire printing machine.
Other image artifacts caused at the fuser roller have also been observed. “Step-and-groove” artifacts are wear-related artifacts, caused by wear of the fuser roller by its repeated fusing of a large number of sheets of one media size (e.g., 8½″ by 11″ paper or transparencies); the subsequent printing of larger media after such wear can result in visible artifacts on the larger media, at locations corresponding to the edges of the worn spots on the fuser roller. Another type of wear-related image artifact is due to wear of a glossy finish roller, which is provided at or after the fuser to impart a glossy finish on selected receiver media. Repeated use of the glossy finish roller for a large number of sheets of one size of media can unevenly wear this roller, causing the glossy finish to be non-uniform when larger media sizes are then printed and fused with a glossy finish.